In January 2007, a brilliant comet, known as Comet McNaught dazzled observers in the Southern Hemisphere. Houstonians missed out on it, though, because of cloudy weather in our area during the brief time that comet was well placed for us. Now, in 2010, a different comet McNaught is becoming visible in our sky at dawn.

Robert H. McNaught, an astronomer at the Australian National Observatory, discovered this comet on September 9, 2009, using a telescope at Australia’s Sliding Spring Observatory. McNaught, a prolific discoverer of comets, has discovered 44 comets (including this one) and is a co-discoverer of 12 others, for a total of 56. This comet’s formal designation is C/2009 R1, where ‘C’ indicates a long period comet and ‘R’ indicates the time of year it was discovered.

Comet McNaught, though, is more than a ‘long-period’ comet. Astronomers have determined that its eccentricity is greater than 1, meaning that its orbit has the shape of a hyperbola. A hyperbolic orbit is the trajectory of a comet that passes near the sun once and never returns. Once McNaught recedes from view, we’ll never see it again.

A hyperbolic orbit also means that McNaught has never been in the inner solar system before. This challenges astronomers who want to predict how it will behave and just how bright it will become in our skies. Already, McNaught is brighter than expected; many expect McNaught to become a naked-eye object by month’s end, especially for those able to observe at a dark site far from light pollution. McNaught is now easily observable in binoculars.

This is a chart from Sky and Telescope, showing the path of Comet McNaught against the background stars. Keep in mind that in June, the stars in this map rise in the northeast just before dawn. McNaught continues to approach the sun until reaching perihelion on July 2, so we expect it to brighten until that date. Unfortunately, a comet near perihelion is generally also close to the sun in our sky, and this comet is no exception. Therefore, McNaught will also get harder to see as it brightens towards the end of the month. After perihelion, McNaught is poorly placed for observers in the Northern Hemisphere.

If you’re looking for the comet on Saturday morning, June 26, you might as well turn around and watch the moon set in partial eclipse. Since the Moon is not precisely aligned with the Earth this time, it will not enter fully into the Earth’s shadow; it goes a little less than halfway in instead. Still, from 5:17 a.m. until moonset at 6:25 a.m., you’ll notice a chunk of the moon’s upper right side missing. (Actually, its the northern limb of the Moon that passes through the shadow. The Moon’s northern limb is on the right as the Moon sets.) The Moon is only about 10 degrees high when the eclipse starts, so you’ll need a southwest horizon clear of tall trees and buildings. Note that the eclipse is still in progress at moonset; we will see less than half of it. Folks far to our west will see a much longer event.